A friction drive device for an omni-directional vehicle that can freely move about on a floor is known. This device comprises a main wheel including an endless annular member and a plurality of free rollers rotatably supported along the outer circumference of the annular member, and a plurality of drive rollers engaging the outer circumferential surfaces of the free rollers at the outer circumferential surfaces thereof so that the rotation of the drive rollers may be frictionally transmitted to the free rollers. See the third embodiment illustrated in FIGS. 17 and 18 of Japanese patent publication No. 3820239B, for instance.
Also is known a wheel for an omni-directional vehicle including two kinds of barrel-shaped free rollers having large and small diameters arranged along the periphery of the wheel in an alternating fashion. Each large diameter barrel-shaped free roller is provided with a recess in which the bearing and a part of the adjacent small diameter barrel-shaped free roller is received, and the curvatures of the two kinds of free rollers are selected such that the overall profile of the barrel-shaped free rollers, and hence the overall profile of the wheel may be highly close to a true circle. See Japanese patent publication No. 3421290B, for instance.